Light imaging, detection and ranging (LIDAR) systems measure distance to a target by illuminating the target with a pulsed laser light and measuring the reflected pulses with a sensor. Time-of-flight measurements can then be used to make a digital 3D-representation of the target. LIDAR systems can be used for a variety of applications where 3D depth images are useful including archaeology, geography, geology, forestry, mapping, construction, medical imaging and military applications, among others. Autonomous vehicles can also use LIDAR for obstacle detection and avoidance as well as vehicle navigation.
Many currently available LIDAR sensors that provide coverage and resolution sufficient for obstacle detection and avoidance in autonomous vehicles are both technologically complex and costly to manufacture. Such sensors can thus be too expensive to allow for wide deployment in mass-market automobiles, trucks and other vehicles. Overall component cost and manufacturing complexity of a particular type of LIDAR sensor is typically driven by the underlying complexities in the architecture of the LIDAR sensor itself. This can be further exacerbated in some modern LIDAR sensors which are a conglomeration of different internal sub-systems, each of which can be in itself quite complex, e.g., optoelectronic systems, electromechanical systems, computer control systems, high-speed communication systems, data processing systems, and the like.
To achieve the high positional accuracy, long distance range, and low power consumption that can be important to some modern sensing applications, stringent technical requirements for each one of these sub-systems have led to architectures and designs that are complex and difficult to build and often require expensive calibration and alignment procedures before individual LIDAR units can be used by a customer. For example, some LIDAR systems have internal architectures that employ one or more large motherboards and bulky, heavy optical systems that are mounted on a counter-balanced structural member, all within a turret that rotates at rates on the order of 1,000 RPM. In some of these systems, separate laser emitter/detector pairs are mounted to individual, separate circuit boards. Thus, each emitter board and receiver board can be required to be separately mounted to the motherboard, with each emitter/detector pair precisely aligned along a particular direction to ensure that the field of view of each detector overlaps with the field of view of the detector's respective emitter. As a result of the above architecture, precision alignment techniques are typically required during assembly to align each emitter board and each receiver board separately.
The above-described architecture becomes increasingly problematic when one desires to scale the resolution of the device. Increasing the resolution requires the addition of more laser emitter/detector pairs, again, each mounted on their own circuit board. Consequently, scaling the resolution linearly with this type of architecture can lead to exponential increases in manufacturing costs and also exponential reductions in reliability due to the sheer number of individual parts and boards involved. Once assembly and alignment is complete, great care must be taken that the precisely aligned multi-board arrangement is not disturbed or jolted out of alignment during shipping or at some other point over the design life of the system.
In addition to the complexities of alignment and assembly of the optical systems, most currently available LIDAR units have a relatively low overall degree of system integration. For example, control and drive circuits in many currently available LIDAR units are separate modules mounted to custom boards. These custom boards may, in turn, need to be mounted to a motherboard within the LIDAR unit or may be mounted somewhere else on a structural element of the LIDAR unit by way of one or more mounting brackets. In some cases, each board can have one or more electrical interconnects that need to be routed through one or more internal volumes or passages within the enclosure to eventually connect with the motherboard.
For rotating LIDAR systems even more additional specialized mounts and interconnects may be required for the electric motor rotor and/or stator. In addition to power connections, data uplink and downlink lines are needed and typically accomplished by one or more inductive, capacitive, and/or metal slip ring rotary couplers, which can be difficult to implement and/or lead to low rates of data transfer. Some systems employ metal brushes within the rotary coupler and are thus potentially unreliable due to the requirement of mechanical contact of the brushes within the rotary mechanism. Other slip ring-type connectors can employ hazardous substances, such as mercury, causing these types of couplers to be non-compliant under the Restriction of Hazardous Substances Directive 2002/95/EC (ROHS) and thus disfavored or even banned in some jurisdictions.
With respect to the optoelectronic systems, the industry has experienced challenges in incorporating cost-effective single photon photodetectors such as CMOS-based single photon avalanche diodes (SPADs) due to their low quantum efficiency in the near infrared wavelengths and their low dynamic range. To improve quantum efficiency, some SPAD-based detectors employ InGaAs technology but such systems are more challenging to integrate in a cost-effective manner than CMOS devices. Therefore, the external/supporting circuitry (e.g., a quenching circuit that can sense the leading edge of the avalanche current, generate a standard output pulse synchronous with the avalanche build-up, quench the avalanche by lowering the bias back down to the breakdown voltage, and then restore the photodiode to the operative level) associated with the SPAD detectors manufactured using InGaAs technology is typically fabricated separately from the SPAD array, for example, in a package that is external to the SPAD array. In addition, InGaAs substrates are relatively expensive and the associated manufacturing processes typically have a lower yield than silicon substrate manufacturing processes further compounding the costs increase. To complicate matters further, InGaAs substrates typically need to be actively cooled in order to reduce dark currents to suitable levels, which increases the amount of energy consumed during runtime, increasing cost and complexity even further.
Rather than employing SPAD-based detectors, many commercially available LIDAR solutions employ avalanche photodiodes (APDs). APDs are not binary detection devices, but rather, output an analog signal (e.g., a current) that is proportional to the light intensity incident on the detector and have high dynamic range as a result. However, APDs must be backed by several additional analog circuits including, for example, analog circuits such as transimpedance amplifiers and/or differential amplifiers, high-speed A/D converters, one or more digital signal processors (DSPs) and the like. Traditional APDs also require high reverse bias voltages not possible with standard CMOS processes. Without mature CMOS, it is difficult to integrate all this analog circuitry onto a single chip with a compact form factor and multiple external circuit modules located on a printed circuit board are usually employed which contributes to the high cost of these existing units.
Accordingly, to support growing markets for 3D sensing systems, there remains a need for more cost effective but still high performing LIDAR systems. Furthermore, there remains a need for improved and more elegant system architectures that enable streamlined assembly processes that can be effectively employed at scale.